The frequent monitoring of blood glucose levels in individuals with diabetes mellitus has become a major factor in the care of such patients over the past decade. Currently, it is possible for the diabetes patient and health care professionals to measure and record blood glucose levels using a variety of portable devices. Due to the need for multiple daily measurements, invasive blood for samples are a burden on the patient and often expensive. As a result, non-invasive devices using spectroscopic techniques, and which are battery powered and use solid-state electronics, have begun to be commercialized. Used at home, these devices allow diabetes patients to monitor and respond to fluctuations in blood glucose on a daily basis.
One example of such a device is disclosed in U.S. Pat. No. 5,070,874 to Barnes, et al. ("the U.S. Pat. No. '874"). As set forth in the U.S. Pat. No. '874, human blood glucose concentration levels vary greatly, and are found within the range of 0-600 milligrams per deciliter (mg/dl). Normal human blood glucose levels are in the approximate range of 80-110 mg/dl. Devices of the type disclosed in the U.S. Pat. No. '874 involve measurement of blood components using near infrared radiation and spectroscopic absorption techniques. Additional devices of this type are disclosed in U.S. Pat. Nos. 5,379,764 and 4,882,492, as well as numerous others, which make use of both reflectance and transmission spectroscopic analysis techniques.
Problems with these prior art devices have resulted due to several issues. One problem is the overlap of the spectrum of glucose with other blood sugars and chemicals. Another relates to hemoglobin-glucose binding, which renders discrete spectral measurements difficult. Also, spectroscopic techniques are typically unable to discriminate between sugars that are metabolized and those that are excreted, resulting in erroneous readings. Still further, prior art devices have failed to address issues which directly impact the accuracy of the measurements taken, such as the spectral effect produced by the skin and tissue, as well as variable blood vessel and skin thickness and composition.
As a result of these and other problems, the repeatability and resulting accuracy of such devices has not been in the range it is desired. The U.S. Food and Drug Administration is currently advising that non-invasive glucose measuring devices should have an accuracy in the range of 15% error or less.